There has been a significant movement toward developing and performing cardiovascular surgeries using a percutaneous approach. Through the use of one or more catheters that are introduced through, for example, the femoral artery, tools and devices can be delivered to a desired area in the cardiovascular system to perform many number of complicated procedures that normally otherwise require an invasive surgical procedure. Such approaches greatly reduce the trauma endured by the patient and can significantly reduce recovery periods. The percutaneous approach is particularly attractive as an alternative to performing open-heart surgery.
Valve replacement surgery provides one example of an area where percutaneous solutions are being developed. A number of diseases result in a thickening, and subsequent immobility or reduced mobility, of heart valve leaflets. Such immobility also may lead to a narrowing, or stenosis, of the passageway through the valve. The increased resistance to blood flow that a stenosed valve presents can eventually lead to heart failure and ultimately death.
Treating valve stenosis or regurgitation has heretofore involved complete removal of the existing native valve through an open-heart procedure followed by the implantation of a prosthetic valve. Naturally, this is a heavily invasive procedure and inflicts great trauma on the body leading usually to great discomfort and considerable recovery time. It is also a sophisticated procedure that requires great expertise and talent to perform.
Historically, such valve replacement surgery has been performed using traditional open-heart surgery where the chest is opened, the heart stopped, the patient placed on cardiopulmonary bypass, the native valve excised and the replacement valve attached. A proposed percutaneous valve replacement alternative method on the other hand, is disclosed in U.S. Pat. No. 6,168,614 (the entire contents of which are hereby incorporated by reference) issued to Andersen et al. In this patent, the prosthetic valve is mounted on a stent that is collapsed to a size that fits within a catheter. The catheter is then inserted into the patient's vasculature and moved so as to position the collapsed stent at the location of the native valve. A deployment mechanism is activated that expands the stent containing the replacement valve against the valve cusps. The expanded structure includes a stent configured to have a valve shape with valve leaflet supports begins to take on the function of the native valve. As a result, a full valve replacement has been achieved but at a significantly reduced physical impact to the patient.
However, this approach has decided shortcomings. One particular drawback with the percutaneous approach disclosed in the Andersen '614 patent is the difficulty in preventing leakage around the perimeter of the new valve after implantation. Since the tissue of the native valve remains within the lumen, there is a strong likelihood that the commissural junctions and fusion points of the valve tissue (as pushed apart and fixed by the stent) will make sealing around the prosthetic valve difficult. In practice, this has often led to severe leakage of blood around the stent apparatus.
Other drawbacks of the Andersen '614 approach pertain to its reliance on stents as support scaffolding for the prosthetic valve. First, stents can create emboli when they expand. Second, stents are typically not effective at trapping the emboli they dislodge, either during or after deployment. Third, stents do not typically conform to the features of the native lumen in which they are placed, making a prosthetic valve housed within a stent subject to paravalvular leakage. Fourth, stents are subject to a tradeoff between strength and compressibility. Fifth, stents cannot be retrieved once deployed. Sixth, stents have an inherent strength that is not adjustable.
As to the first drawback, stents usually fall into one of two categories: self-expanding stents and balloon expandable stents. Self-expanding stents are compressed when loaded into a catheter and expand to their original, non-compressed size when released from the catheter. These are typically made of Nitinol. Balloon expandable stents are loaded into a catheter in a compressed but relaxed state. These are typically made from stainless steel or other malleable metals. A balloon is placed within the stent. Upon deployment, the catheter is retracted and the balloon inflated, thereby expanding the stent to a desired size. Both of these stent types exhibit significant force upon expansion. The force is usually strong enough to crack or deform thrombosis, thereby causing pieces of atherosclerotic plaque to dislodge and become emboli. If the stent is being implanted to treat a stenosed vessel, a certain degree of such expansion is desirable. However, if the stent is merely being implanted to displace native valves, less force may be desirable to reduce the chance of creating emboli. An additional concern related to displacing an aortic valve is the risk of conduction disturbances (i.e. left bundle branch block) due to the close proximity of the conduction pathways to the native valve structure. Excessive radial force applied at the native valve site increases the risk of irritation or damage to the conduction pathway and heart block.
As to the second drawback, if emboli are created, expanded stents usually have members that are too spaced apart to be effective to trap any dislodged material. Often, secondary precautions must be taken including the use of nets and irrigation ports.
The third drawback is due to the relative inflexibility of stents. Stents typically rely on the elastic nature of the native vessel to conform around the stent. Stents used to open a restricted vessel do not require a seal between the vessel and the stent. However, when using a stent to displace native valves and house a prosthetic valve, a seal between the stent and the vessel is necessary to prevent paravalvular leakage. Due to the non-conforming nature of stents, this seal is hard to achieve, especially when displacing stenosed valve leaflets.
The fourth drawback is the tradeoff between compressibility and strength. Stents are made stronger or larger by manufacturing them with thicker members. Stronger stents are thus not as compressible as weaker stents. Most stents suitable for use in a valve are not compressible enough to be placed in a thin catheter, such as a 18 Fr catheter. Larger delivery catheters are more difficult to maneuver to a target area and also result in more trauma to the patient.
The fifth drawback of stents is that they are not easily retrievable. Once deployed, a stent may not be recompressed and drawn back into the catheter for repositioning due to the non-elastic deformation (stainless steel) or the radial force required to maintain the stent in place (Nitinol). Thus, if a physician is unsatisfied with the deployed location or orientation of a stent, there is little he or she can do to correct the problem.
The sixth drawback listed above is that stents have an inherent strength and are thus not adjustable. As previously stated, stronger stents are made with stronger members. Once a stent is selected and deployed, there is little a physician can do if the stent proves to be too strong or too weak.
Various embodiments of devices that solve these problems are introduced in U.S. Patent Publication No. 2006/0271166 to Thill et al., entitled “Stentless Support Structure,” the contents of which is incorporated herein in their entirety. This publication teaches a braided mesh tube that is capable of folding back and forth into itself to build, in situ, a support structure that is strong enough to hold back the leaflets of a native valve sufficiently to successfully deploy a replacement valve, thus obviating the need for excision of the native valve. Advantageously, because of the inverting nature of these devices, the braided mesh, in an elongated delivery configuration, does not need to possess the strength to accomplish native valve displacement until the inversion process occurs. This allows the mesh tube to be constructed such that, in the elongated delivery state, the tube can be compressed into a very small catheter, such as a 18 Fr or smaller catheter. Such a small catheter significantly reduces patient trauma and allows for easy percutaneous, intraluminal navigation through the blood vessels. It is to be understood that terms like transluminal and percutaneous, as used herein, are expressly defined as navigation to a target location through and axially along the lumen of a blood vessel or blood vessels as opposed to surgically cutting the target vessel or heart open and installing the device manually. It is further to be understood that the term “mesh” as used herein describes a material constructed of one or more braided or woven strands.
In order to accomplish the folding back and forth feature of this device, there are preformed, circumferential folds in the device. One embodiment has two circumferential folds that are longitudinally spaced apart in the extended configuration. One of these folds is preformed to fold inwardly, and the other is preformed to fold outwardly. These preformed folds, when released out of a catheter, tend to return to a folded configuration that has a z-like cross-section. This cross-section design results not only because the inward pre-formed fold folds inwardly and the outward pre-formed fold folds outwardly, but because these folds reverse longitudinal positions once folded. If the inward preformed fold is distal of the outward preformed fold in the extended position, in the folded position the inward preformed fold will be proximal of the outward preformed fold. This design allows a valve on a distal end of the device to be drawn into the device when folded, without requiring the valve itself to be inverted or everted. In one embodiment having two preformed folds, the inversion process thus results in a three-layered configuration that could be significantly shorter than the extended length, depending on the spacing of the folds.
In the development of the devices described in the aforementioned publication, U.S. Pat. Pub. 2006/0271166, it was found that, occasionally, it was advantageous to use an additional device to hold the outermost layer of the implant axially in place while inversion of a layer was being effected. This gave rise to the delivery tool shown and described in U.S. Patent Publication 2008/0082165 to Wilson et al., entitled “Delivery Tool For Percutaneous Delivery Of A Prosthesis.” This delivery tool includes an expandable mesh region that, when axially compressed, flares outwardly to form a bulbous or rounded structure of increased radius. Further axial compression creates a flat, disc-like surface. In use, the device is extended through an implant prior to releasing the implant from the delivery catheter. The device is then expanded to the disc-like configuration and pulled proximally to act as a backstop at a desired target location, against which the implant is delivered. Thus, the disc-like device prevents axial migration of the implant in a distal direction if and when distal force is placed on the implant during inversion of the second or subsequent layers into the first layer.
It has been found, however, that in some cases, depending on target location and patient anatomy, there is insufficient space in a distal axial direction beyond the target location to efficiently use this delivery device. For example, some patients may have limited left ventricular space, which may prohibit the use of the backstop device.
There is thus a need for a device that is able to prevent axial migration of the aforementioned braided implant devices during inversion, but does not require significant space distally beyond the target location.